Occurrence and Geochemistry of the K-rich Brines in Deep-Seated Aquifers in the Sichuan Basin

Subsurface K-rich brines (K contents ranging from 1.5 to 53 g/L) are found in deep-seated aquifers in the Sichuan Basin of China. Most of the K-rich brines occur in the marine carbonate rocks of the Middle and Lower Triassic, Middle Cambrian and Sinian, and only small amounts of the K-rich brines are found in the continental Upper Triassic sandstone aquifers. The brine-bearing formations are at depth ranging from 300 to 4650 m and the brines are rich in anticlines and fault zones. The subsurface brines occur under a sealed state and do not receive any recharge. The brines have high TDS ranging from 80 to 388 g/L with relatively high concentrations of Br, I, Li, Sr, Ba, and B. The K-rick brines are mainly of Cl-Na and Cl-Na·Ca type. In particular, the PL4 well tapping the Middle Triassic brine-bearing carbonate rocks in the western part of the basin flows brines having TDS in the ranges of 348.392 to 377.27-387.78 g/L, which are close to those of the Yellow Sea water and the South China Sea water when evaporated to the stage of epsomite precipitation. K contents of the brines in the PL4 well range from 48.95 to 53.267-53.38 g/L and the B contents are as high as 4.994 g/L, which are much higher than those of the evaporated sea water at the stage of epsomite precipitation. Hydrogen and oxygen isotopes of the brines indicate that the K-rick brines in the carbonate rocks are of marine origin and those in the sandstone aquifers are of meteoric origin.